1. Field of the Invention
The present invention is directed to a power amplifier of the type having multiple, switched output stages and containing a driver which operates the output stages. The present invention is also directed to a driver for such a power amplifier, and to a method for operating such a power amplifier. The invention is preferably utilized in a gradient amplifier of a nuclear magnetic resonance tomography apparatus that has a number of series-connected switched output stages for driving a gradient coil.
2. Description of the Prior Art
German OS 43 04 517 corresponding to U.S. Pat. No. 5,546,299, discloses a power supply for an inductive load wherein two switched output stages are connected in series at the output side. A control arrangement composed of two control modules generates drive signals for active switch elements of the output stages.
During operation, the output stages apply a voltage of, for example, several hundred volts to the gradient coil in order to periodically build up and dismantle an exactly regulated current of, for example, 300 A. Given a number of coupled output stages, operating conditions are possible in which an energy transfer between the output stages occurs.
When, for example, the predetermined shape of the current curve exhibits steep leading edges and flatter trailing edges and the output stages assume the function of a basic load output stage and a peak load output stage, it can occur that the peak load output stage is only cut in during the steep leading edges of the current. The energy supplied as an output by the peak load output stage to the gradient coil is then returned exclusively to the basic load output stage during the flatter trailing edges of the current. This can lead to an excessive charging (over-voltage) of a charging capacitor in the basic load output stage, and thus to a malfunction of or damage to the gradient amplifier.
Such an undesired charging of an output stage is also possible when the output stages are differently driven (for example, supply output voltages of different polarity), when they exhibit different properties (for example, in view of clock frequency or intermediate circuit voltage) or when a unit known as a booster is connected to the load circuit.
As a first alternative for avoiding over-voltages, German OS 43 04 517 and U.S. Pat. No. 5,546,299 disclose the use of discharge resistors that are connected parallel to the charging capacitors. The energy consumption and the dissipated heat of the output stages, which must be conveyed away with a cooling arrangement increase substantially due to such discharge resistors. As a second alternative, German OS 43 04 517 and U.S. Pat. No. 5,546,299 employ switchable discharge means (DC--DC converters) for the energy distribution between the output stages. Such discharge means, however, are complicated in terms of their realization, particularly when more than two output stages are provided.